-- Last Updated: Nov-26-12 5:42 PM EST --
It is common to provide a downward slope in the forward direction, especially to provide comfort when kneeling (though 1.0 to 1.5 inches of height differential would be more common when setting up a seat for kneeling). Most canoes I've looked at have no noticeable difference in gunwale height occurring within the seat locations, but maybe I haven't looked closely enough. Usually, most of the upward angle of the gunwales occurs closer to the ends of the boat than where the seats are located, though the rear seat is more likely to be within that zone than the front seat (it's closer to its end of the boat than the front seat is to its end).
I think I've seen some aluminum canoes having the rear seat higher than the front, but I can't say I've noticed a trend for that. I bet it's common though, as I doubt it's normal to use different hangers for the front versus the back. Someone else may prove me wrong though.
Wood-truss style hangers are easy to work with. Just trim them gradually, trying the boat in-between adjustments, until you get it the way you like.